Listeria monocytogenes

Here we explain what listeria is and how you can prevent getting it

Listeria

Listeria causes a rare infection called listeriosis.  It is usually caught from eating food containing the listeria bacteria.  It is an important bacteria to be aware of, not because of the number of people each year affected but due to the severity of infection and the loss of life it causes.  It can cause serious consequences in pregnant women and their unborn babies, elderly adults and people with weakened immune systems.

What happens when you get infected with listeria

Due to the long incubation period for Listeria monocytogenes (up to 90 days) it is difficult to determine the key sources of infection.  Healthy adults are likely to experience only mild infection, causing flu-like symptoms or gastroenteritis for a few days, such as:

  • high temperature
  • aches and pains
  • chills
  • feeling sick or vomiting
  • diarrhoea

However, those most at risk of severe illness are:

  • pregnant women
  • newborn babies
  • the elderly
  • people with weakened immune systems, such as cancer patients

There is more information on the NHS choices website if you need to find out more clinical information.

Preventing listeria food poisoning

Listeria is widespread in the environment however most cases of Listeria are due to contaminated food.  Listeria is an unusual bacterium because it can grow at low temperatures, including refrigeration temperatures of below 5°C. It is, however, killed by cooking food thoroughly and by pasteurisation.

A wide range of foods can become contaminated with listeria, it is of most concern in chilled ready-to-eat foods that do not require further cooking or re-heating:

  • cooked sliced meats
  • cured meats
  • smoked fish
  • cooked shellfish
  • blue veined and mould-ripened soft cheeses
  • pate
  • pre-prepared sandwiches and salads
  • unpasteurised cheeses

To reduce the risk of listeria when preparing food at home, it’s important to:

  • keep chilled ready to eat foods cold – make sure your fridge is working properly and is set to 5°C or below
  • always use foods by their use by date
  • follow the storage instructions on the label and use opened foods within 2 days unless instructions on the packaging say otherwise
  • ready-to eat foods must be eaten within 4 hours of being taken out of the fridge
  • ensure that you follow manufacturer’s instructions in the preparation of all foods

Under the microscope - use-by-dates and how they are set

Listeria can grow at fridge temperatures so a food business must put controls in place to minimise the shelf life will limit the opportunity for listeria to grow to harmful levels. That is why there are use-by-dates on perishable foods and why they need to be adhered to.

One of the ways a food business can do this is to identify any potential hazards they may have in the product and take it to a food laboratory to have its safety checked. 

The product will be tested at various points in the shelf life process e.g. if the food business believes their food will be safe to eat up to 6 months after purchase they have to test it at frequent intervals up to 6 months to see if that is true.

Even once a food product has a use-by-date set the business has to routinely check that the food is safe with that date.

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